CMP 131Introduction to Computer

Programming

Violetta Cavalli-Sforza

Week 5, Lecture 2 (Tuesday)

TODAY

• Input/Output and Formatting

• Your questions on everything before the Quiz tomorrow

Input/Output (Read/Write) Procedures

• Definitions:– Input Operation:

• An instruction that reads data into memory.– Output Operation:

• An instruction that displays information stored in memory.

– Procedure:• A piece of code that is invoked from within the

program and returns control to the program when finished

– I/O Procedure:• Pascal procedure that performs I/O operation.

– Procedure Call Statement:• An instruction that calls or activates a

procedure.

Input/Output (Read/Write) Procedures

• Data can be stored in memory in three different ways:– Associated as a constant– Assigned to a variable– Read into a variable

• Reading is used if the data varies each time the program executes.

• Input/output procedures are supplied as part of Pascal compiler. Their names are standard identifiers.

The Same Identifier has Different Meanings in Different Places

• When a variable name occurs in a read/readln and a write/writeln statement, it has a different meaning:– write(MyVar) : MyVar is an expression

The arguments to write are expressions (literals, constants, variables, combinations) whose value is retrieved for display

– read(MyVar) : MyVar is interpreted as the address of a memory location to read into.You cannot have an expression as an argument to read

• Similarly, when a variable name occurs in an assignment statement, it has a different value on the right hand side (RHS) of the := and on the left hand side of the :=– On the RHS, it is (part of) an expression whose value is being

retrieved– On the LHS, it is the address of a memory location where the

RHS value will be stored.

Reading Data …• Procedures: read, readln• Read data from standard input device “input”

(e.g. keyboard)• Syntax:

read/readln(inloc1, inloc2, …, inlocN)inlocX is a location where to store the data

• Examples:read (SqMeters);readln (Letter1, Letter2, Letter3);read (Age, FirstInitial)

• Commas separate the variable names in input list

Reading Data …

• Use write/writeln to display a prompt message when the user is required to enter data– Otherwise the program is waiting and you don’t know why or

what to enter.

• The order of data enter must correspond to the order of variables in the input list

• Reading numeric data:– Insert one or more blanks between numeric data items

• Reading character or string data:– Don't insert any blanks between consecutive data items unless

the blank character is one of the data items being read

Read vs. Readline Procedure

• Read:– Will read from input just as much as it needs for its

arguments and leave the rest to be consumed later.– Extra characters on the data line after a read

operation are not read until the next read or readln operation

• Readln– Will read from input just as much as it needs for its

arguments – It waits for a Return/Enter key character – Extra characters on the data line between what is

needed and the end of line will be ignored after ReadLn is finished

PROGRAM ReadTest;

VAR x,y: integer;

BEGIN

write('Type 2 integers separated by a space > ');

read(x); writeln; writeln('Read x ',x);

read(y); writeln; writeln('Read y ',y);

writeln('Type ENTER to exit program.'); readln;

END.

PROGRAM ReadLnTest; VAR x,y,z: integer;BEGIN writeln('Type 2 or more integers separated by a space.'); writeln('Terminate your input by pressing ENTER.'); readln(x,y); writeln('Read x ',x,' and y ',y); read(z); writeln('Read z ',z);END.

Writing/Displaying Data

• Procedures: write, writeln• Write data to standard output device “output” (e.g.

terminal)• Syntax:

write/writeln(expr1, expr2, …, exprN)where exprX is an expression, whose value is to be written, with optional formatting information

• Examples:writeln (Letter1, Letter2, Letter3);write (‘Age = ’, Age, ‘First Initial = ’,FirstInitial)write (SqMeters:10:2);

• Commas separate the variable names in output list• Writes every argument in the output list in the order in

which it is given.

write vs. writeln Procedure

• writeln – used to display a line of program output,

including a newline at the end; cursor advances to the next line after the output is displayed.

– without any output list instructs the output device to advance by a blank line

• write– Cursor doesn't advance to the next line after

the output is displayed.

Self-Check

• What is the output if the entered data are 5 7?writeln (‘Enter two integers>’);readln(M, N);M := M + 5;N := 3 + N;writeln (‘M = ‘, M);writeln (‘N = ‘, N);

• What of the output of this code?write (‘My name is’);writeln (‘Doe, Jane’);writeln ;write(‘I live in ‘);write(‘Ann Arbor, MI’);writeln (‘and my zip code is ‘, 48109);

No Formatting with write/writeln• Integer values: take the space they need• Real values: Most Pascal compilers use

scientific notation (floating-point notation) to display real values, but you can change that with formatting directives.

• Character values display as themselves, without quotes

• Boolean values display as FALSE and TRUE with no quotes

• String literals, the characters inside the quotes are printed, but not the quotes themselves

PROGRAM TestOutputNoFormat;

BEGIN

writeln('An integer with no formatting : ', 45);

writeln('A real with no formatting: ', 45.67);

writeln('A character with no formatting: ', '#');

writeln('Boolean values, no formatting : ', false, ' and ', true);

END.

Formatting Integer Output Formatting Integer Values – b stands for blank space

Value Format Printed Output234 :4 b234

:5 bb234:1 234:Len bb234 (if

Len=5)-234 :4 -234

:5 b-234:1 -234

Formatting Integer Output

• Add the symbol ‘:n’ to the integer output list item, where n specifies the number of digits to be displayed (field width).

• The digits will be right-justified.

• For negative numbers, the minus sign is included in the counts of digits displayed

The format specification ‘:1’ can always be used to display any integer values without leading blanks.

The field width specification may be a variable, or even an expression

The syntax for each integer item in an output list is: <integer expression>:<integer expression>

PROGRAM TestOutputIntFormat; VAR Dollars, Cents : integer;BEGIN Dollars := 5; Cents := 33; writeln('Your collection was worth '); writeln(Dollars : 1, ' dollars', ' and ', Cents :1, ' cents. '); writeln(Dollars : 2, ' dollars', ' and ', Cents :2, ' cents. '); writeln(Dollars : 3, ' dollars', ' and ', Cents :3, ' cents. '); writeln(Dollars : 4, ' dollars', ' and ', Cents :4, ' cents. '); writeln(Dollars : 5, ' dollars', ' and ', Cents :5, ' cents. ');

Dollars := -2; Cents := 89; writeln('but is now worth '); writeln(Dollars : 1, ' dollars', ' and ', Cents :1, ' cents. '); writeln(Dollars : 2, ' dollars', ' and ', Cents :2, ' cents. '); writeln(Dollars : 3, ' dollars', ' and ', Cents :3, ' cents. '); writeln(Dollars : 4, ' dollars', ' and ', Cents :4, ' cents. '); writeln(Dollars : 5, ' dollars', ' and ', Cents :5, ' cents. ');END.

Formatting Real Output

• Examples: b stands for blank space

Value Format Printed Output3.14159 :5:2 b3.14

:4:2 3.14:3:2 3.14:5:1 bb3.1:8:5 b3.14159:9 3.142E+00

-3.14159 :9 -3.14E+00 -124.3163 :4:2 -124.32

Formatting Real Output

• Total field width & desired number of decimal places should be indicated.

• Add “:n:d“ to the integer.– n = total field width– d = number of decimal digits.

• Total field width should be large enough to accommodate all digits before and after the decimal point – Pascal will use the extra space it needs anyhow so your nicely

formatted data may not look so well formatted after all.• Decimal point & sign for negative numbers are included

in the count.• It is possible to use the form “:n“ for real numbers. In

this case, the real value is printed in scientific notation using a total of n print positions.

Formatting Real Output

• Eliminating Leading Blanks:– Choose a format that will display the smallest value

expected without leading blanks..– Examples:

29397 :1 is displayed as2939799397.567 :3:1 is displayed as99397.6

– To display a real value without leading blanks, use format specification ‘:3:1’ for one decimal place or ‘:4:2’ for two decimal places.

– If you use ‘:n’, the scientific notation will be displayed.

Formatting Character & String Output

• Formatting Strings:– A string value is always printed right-justified in its field.– If the field width is too small to accommodate the string value,

Pascal will take the space it needs.

• Examples: b stands for blank spaceString Format Printed Value'*' :1 *

:2 b*'ACES' :1 ACES

:2 ACES:3 ACES:4 ACES:5 bACES

PROGRAM TestOutputStringFormat;BEGIN writeln('ACES':1); writeln('ACES':2); writeln('ACES':3); writeln('ACES':4); writeln('ACES':5); writeln('ACES':6);END.

Self-Check• Show how the value -15.564 (stored in X)

would be displayed using the following formatsX :8:4 X:8:3 X:8:1 X:8:0 X:8

• Assuming X = 12.345 (type Real) and I = 100 (type Integer). What will the output of the following statements look like?

writeln(‘X is ‘ :10, X :6:2, ‘I is ‘ :4, I :5);write(‘I is ‘ :10, I : 1);writeln(‘X is ‘ :10, X :2:1);